Conventional martensitic stainless steels usually contain 10.5% to 13% chromium and up to 0.25% carbon. Precipitation hardening martensitic stainless grades contain up to 17% chromium. Chromium, when dissolved in solid solution, provides the corrosion resistance characteristic of stainless steels. Many martensitic stainless steels also contain (i) ferrite stabilizing elements such as molybdenum, tungsten, vanadium, and/or niobium to increase strength; (ii) austenite stabilizing elements such as nickel and manganese to minimize delta ferrite formation and getter sulfur, respectively; and (iii) deoxidizing elements, such as aluminum and silicon, Copper is sometimes present in precipitation hardening martensitic stainless grades.
Conventional martensitic stainless steels are usually hot worked to their final shape, then heat treated to impart an attractive combination of mechanical properties, e.g., high strength and good toughness, within limited attainable ranges. Typical heat treatment of conventional martensitic stainless steels involves soaking the steel between approximately 950° C. and 1100° C. and air cooling (“normalizing”), oil quenching, or water quenching to room temperature, and subsequently tempering the steel usually between 560° C. and 750° C. Tempering of conventional martensitic stainless steels results in the precipitation of nearly all carbon as chromium-rich carbides (i.e., M23C6) and other alloy carbides (e.g., M6C) which generally precipitate on martensite lath boundaries and prior austenite grain boundaries in the body-centered-cubic or body-centered-tetragonal ferrite matrix. (“M” represents a combination of various metal atoms, such as chromium, molybdenum and iron.)
In 12-13% Cr steels, approximately 18 of the 23 metal atoms in M23C6 particles are chromium atoms. Thus, for every 6 carbon atoms that precipitate in M23C6 particles, approximately 18 chromium atoms also precipitate (a carbon to chromium atomic ratio of 1:3). The volume fraction of M23C6 precipitates is usually proportional to the carbon content. Therefore, in a 12% Cr steel with 0.21 wt % carbon (which equals approximately 1 atom % carbon), about 3 wt % chromium (˜3 atom % chromium) precipitates as M23C6 particles, leaving an average of about 9 wt. % chromium dissolved in solid solution in the matrix. If this material were tempered at a relatively high temperature, the chromium remaining in solid solution (˜9%) would be uniformly distributed in the matrix due to thermal atomic diffusion. However, if the tempering temperature is relatively low and diffusion is sluggish, regions surrounding the M23C6 precipitates will contain less chromium than regions further away from the particles. This heterogeneous distribution of chromium in solid solution is known as sensitization and can cause accelerated localized corrosion in chromium-lean areas immediately surrounding the M23C6 particles. To preclude sensitization of conventional 12% Cr steels with relatively high carbon contents, high tempering temperatures are used. However, the yield strength (0.2% offset) of conventional martensitic stainless steels is reduced after tempering at high temperatures—generally to less than 760 MPa.
Several martensitic stainless steels have been developed that contain low levels of carbon (<0.02 wt. %) and relatively high amounts of nickel and other solid solution strengthening elements, such as molybdenum. Although these low carbon martensitic stainless steels are not generally susceptible to sensitization, they can be heat treated to yield strengths only up to about 900 MPa. Moreover, the cost of these steels is relatively high, primarily because of the large amounts of expensive nickel and molybdenum in them.
U.S. Pat. No. 5,310,431, issued to the present inventor, discloses “an iron-based, corrosion-resistant, precipitation strengthened, martensitic steel essentially free of delta ferrite for use at high temperatures has a nominal composition of 0.05-0.1 C, 8-12 Cr, 1-5 Co, 0.52-2.0 Ni, 0.41-1.0 Mo, 0.1-0.5 Ti, and the balance iron. This steel is different from other corrosion-resistant martensitic steels because its microstructure consists of a uniform dispersion of fine particles, which are very closely spaced, and which do not coarsen at high temperatures. Thus at high temperatures this steel combines the excellent creep strength of dispersion-strengthened steels, with the ease of fabricability afforded by precipitation hardenable steels.” U.S. Pat. No. 5,310,431 is herein incorporated by reference in its entirety.